Zinc oxide (ZnO) is applied to, for example, a pyroelectric element, a piezoelectric device, a gas sensor and a transparent electrically conductive film, but zinc oxide is a direct transition-type semiconductor having a forbidden band width of 3.4 eV and is promising as a material for an LED in the blue to ultraviolet region or other photoelectric devices.
Regarding the conventionally employed hydrothermal synthesis method or flux method, not only a large single crystal is difficult to produce, but also the hydrothermal synthesis method requires a special production apparatus that generates a high temperature and a high pressure and is disadvantageous in view of cost. A molecular beam epitaxy method and the like have been also proposed, and such a method is suitable for the production of a single crystal thin film, but not for the production of a large single crystal. Under these circumstances, a melt pulling method using vanadium oxide or molybdenum oxide as the solvent with boron oxide has been proposed (see, Japanese Unexamined Patent Publication (Kokai) Nos. 2002-193698 and 2003-2790).
In the context of the present invention, the term “solute” is zinc oxide, the term “solvent” is the substance for melting the solute and containing one or more kinds of compounds such as an oxide or halide (e.g., fluoride, chloride), and the “melt” is a state where the solute is melted in the solvent, and depending on the case, includes a state where a part of the melted solute is precipitated and a solid and a liquid are present together. The liquid part in this case is referred to as a “liquid phase” for differentiating it from the melt.
In Japanese Unexamined Patent Publication (Kokai) No. 2002-193698, zinc oxide as the solute and vanadium oxide and/or boron oxide as the solvent are mixed and melted under heating and thereafter, the melt temperature is lowered to deposit and grow a fine crystal of zinc oxide ZnO on a seed crystal or a substrate. At this time, the temperature in the part of the seed crystal contacting the melt becomes lower by several tens of ° C. than the melt temperature because heat is dissipated from a rod to which the seed crystal is fixed. The crystal of zinc oxide is thereby selectively deposited on the seed crystal. In Japanese Unexamined Patent Publication (Kokai) No 2003-2790, zinc oxide as the solute and molybdenum oxide as the solvent are mixed and melted under heating and thereafter, the melt temperature is maintained at a fixed temperature or lowered to deposit and grow a fine crystal of zinc oxide ZnO on a seed crystal or a substrate.